Boston police announced that an FBI news conference, which had been postponed twice Wednesday, would occur at 8 p.m. ET. But a few minutes later, the news conference was canceled.

CNN and the Associated Press had reported that a suspect was arrested early Wednesday afternoon, but a representative for the FBI, in an email to Hearst Television's Washington News Bureau, said that no arrest had been made. CNN retracted its report, while AP said its source "stood by the information even after it was disputed."

Boston Police also said no arrest had been made, but WCVB in Boston reported that its sources said an arrest was imminent.

WCVB reported that surveillance video taken by cameras at Lord & Taylor on Boylston Street helped authorities identify a suspect placing a bomb. AP reported Wednesday afternoon that authorities have an image of the potential suspect, but did not have the man's name.

A bomb threat forced the evacuation of the Moakley Federal Courthouse Wednesday at about 3 p.m. The courthouse is where any suspect would be brought to be arraigned on charges related to the bombing.

WCVB reporter Kelley Tuthill was inside the courthouse at the time and described someone on a loudspeaker issuing a "code red." The evacuation lasted about an hour before employees were allowed back inside, Tuthill reported.

Boston Mayor Thomas Menino confirmed that the investigation has made significant progress Wednesday.

Authorities investigating the deadly bombings at the Boston Marathon have recovered a piece of circuit board that they believe was part of one of the explosive devices, and also found the lid of a pressure cooker that apparently was catapulted onto the roof of a nearby building, an official said Wednesday.

A law enforcement official briefed on the investigation confirmed to AP that authorities have recovered what they believe are some of the pieces of the explosive devices. The official spoke on condition of anonymity because this person was not authorized to publicly discuss evidence in the ongoing investigation.

Earlier, a federal law enforcement source with firsthand knowledge of the investigation told CNN that a lid to a pressure cooker thought to have been used in the bombings had been found on a roof of a building near the scene.

A person close to the investigation previously told AP the bombs consisted of explosives put in 1.6-gallon pressure cookers, one with shards of metal and ball bearings, the other with nails.

An unclassified bulletin was sent Tuesday evening to law enforcement agencies across the country and said that the bomb inside the backpack was an “improvised explosive device (IED)” made out of a common pressure cooker.

The FBI says the other explosive was in a metal container, but there wasn't enough evidence to indicate that it was a pressure cooker.

It was not known what was used to set off the two explosives that killed three people Monday and injured more than 170 others.

Monday's bombings killed 8-year-old Martin Richard, of Boston, 29-year-old Krystle Campbell, of Medford, Mass., and Lu Lingzi, a Boston University graduate student from China. More than 170 others were wounded in the attack.

President Barack Obama is expected to travel to Boston on Thursday to attend a memorial service for the victims.